The Frozen Sea is the large body of water on the southern coast of Northrend.
In Wrath of the Lich King
This section concerns content exclusive to Wrath of the Lich King.
The Frozen Sea zone surrounds Northrend's coastal zones. It has Deep Sea Monsterbelly Schools and Moonglow Cuttlefish Schools. Outside of pools, Deep Sea Monsterbellies, Rockfin Groupers and Moonglow Cuttlefish can be caught. The only quest which takes place in the zone is Monsterbelly Appetite, a daily fishing quest. It has no mining or herb nodes.
According to some sources, the Frozen Sea requires the highest fishing skill in the game to catch any fish (minimum 480, or above the unmodified maximum of 450).